Preview OpenType Features in FontForge Metrics View

This guide explains how to preview and test OpenType layout features—such as kerning, ligatures, contextual alternates, and small caps—directly within FontForge's Metrics View. By properly configuring your script, language, and lookup tables, you can interactively verify how your OpenType substitutions and positioning rules behave in real-time text strings before exporting the final font file.

Step 1: Open the Metrics Window

Launch FontForge and load your font project. Open the Metrics View by selecting Metrics from the main menu bar and choosing New Metrics Window, or by using the shortcut Ctrl + K (or Cmd + K on macOS).

Step 2: Input Your Test String

At the top of the Metrics View, enter the sequence of characters designed to trigger your OpenType features into the text field. For example, type "fi" or "ffi" to test standard ligatures (liga), or specific letter pairs (such as "AV" or "To") to inspect pair kerning (kern).

Step 3: Set the Script and Language System

OpenType layout features are tied to specific scripts and languages. Inside the Metrics Window:

  1. Navigate to the View menu.
  2. Select Script and choose the appropriate script (e.g., latn for Latin).
  3. Select Language and pick the appropriate language tag (e.g., dflt for default, or specific tags like ENG or TRK).

If the active script or language does not match the rules defined in your font's lookup tables, the features will not execute in the preview.

Step 4: Toggle OpenType Layout Features

To see substitutions (GSUB) and positioning adjustments (GPOS) applied to your text:

  1. In the Metrics Window menu, click on View.
  2. Select Features... to open the feature activation list.
  3. Check the boxes next to the OpenType tags you wish to preview (e.g., liga, dlig, kern, calt, smcp).
  4. Click OK or apply the selection.

The rendered glyphs in the Metrics Window will immediately update to reflect the active rules. You can toggle these features on and off to compare the default glyph shapes and metrics against the feature-applied versions.

Troubleshooting Missing Features

If your features do not render in the Metrics View:

  • Verify Lookups: Go to Element > Font Info > Lookups to confirm that your substitutions and positioning rules are correctly assigned to the appropriate OpenType feature tags and scripts.
  • Glyph Names: Ensure the glyph names mapped within your substitution rules accurately match the names of the glyphs present in your font database.
  • Order of Execution: Check that lookups in the Lookups tab are listed in the correct logical sequence, as FontForge processes them sequentially.